1. Field of the Invention
The invention relates to a process for draining a fibrous pulp suspension that includes pre-draining the fibrous pulp suspension in an open top container, disposing the fibrous pulp suspension against a moving, perforated first face, which, due to different pressures acting on opposite sides of the first face is thus drained, and transporting the pre-drained suspension upwardly out of the fibrous pulp suspension for further draining. The invention also relates to a drum thickener for the multi-stage draining of a fibrous pulp suspension.
2. Discussion of Background Information
Processes of this kind are used in order to either increase the pulp density of the fibrous pulp suspension or to remove undesirable components from the fibrous pulp suspension by washing. Both processes are more or less strongly connected with each other because draining is naturally necessary for the washing of a fibrous pulp suspension. On the other hand, though, the extent of the washing effect is a function of other parameters, i.e. not only the increase of the pulp density. Processes of the type mentioned above can be carried out, for example, with known devices for draining a fibrous pulp suspension, the so-called drum thickeners. These form a fibrous web on the outside of a drum that dips into the suspension and this fibrous web is drained into the interior of the drum. Due to the circumferential movement of a drum of this kind, the fibrous pulp web is transported out of the suspension container and can be removed as a thickened pulp. It is then often characterized as accepted stock. Steps have also already been carried out for further increasing the pulp density of the pulp that has already come out of the suspension and is still adhering to the drum. To this end, in particular, so called couch rolls are used, which produce an additional pressure on the fibrous pulp web. Or one presses, a revolving belt against the web, e.g. according to U.S. Pat. No. 2,278,525 or WO 94/08088. As a rule, measures of this kind do achieve a further increase of the pulp density, but this is often still too slight and is also partially reduced again, e.g. by remoistening upon emerging from the nip.
In a drum thickener according to U.S. Pat. No. 1,421,364, the vat is completely filled with suspension during operation. The polygonal outer surface of the drum conveys the thickened pulp out along arched sealing faces.
WO 96/08600 describes another draining device. This likewise contains a rotating drum with a permeable surface. On a part of the circumference, the drum is encompassed by an impermeable plastic belt. The suspension is sprayed into the gap formed between the drum and plastic belt with the aid of a headbox. A vacuum is applied from the inside in this region, in which the pulp jet reaches the drum wall. This machine is very expensive to produce and operate.
U.S. Pat. No. 4,750,340 discloses a device suitable for thickening, with a rotary driven, perforated drum which is surrounded by an arched plate fixed to the drum in a slightly eccentric manner. As a result, a nip is formed between the drum and the wall, into which already pre-thickened pulp is introduced by way of a feed worm. The device is very costly and is particularly suited for higher initial pulp densities.